The present invention relates to the field of abrasive grinding, and more particularly to the field of grinding wheels having a hub for mounting the wheel to a grinding machine.
The many grinding applications employing an abrasive wheel mounted on a grinding machine has led to the development of a wide variety of wheel shapes and sizes. One type of grinding wheel tailored for specific applications is the depressed center wheel, characterized by the central portion of the wheel being laterally offset from the wheel periphery so that one face has a depressed central portion and the other has a raised central portion. This design allows a user to perform face grinding operations, using the wheel face of the side having the depressed center. Typically such operations are performed on metal, masonry or concrete surfaces and the like, using portable grinding machines.
Safety, of course, is a primary consideration in this art; wheels typically operate at high rotational speeds, and failure can cause an "explosion" of the wheel, resulting in portions of abrasive material flying in many directions at high velocity. Therefore, the art has adopted uniform design and safety standards, published as American National Standards Institute (ANSI) Standard B7.1, hereby incorporated herein by reference. Depressed center wheels are classified as ANSI Types 27 and 28.
The means by which a grinding wheel is secured to the grinding machine spindle is important generally, and of particular significance to depressed center wheels. In general, the mounting means must be capable of holding the wheel exactly perpendicular to the spindle, and it must provide support to the wheel to distribute stresses away from the central mounting aperture, where stresses are at a maximum. The mounting means also must be firmly secured to the abrasive wheel, to prevent any slippage. In addition, the mounting means for Types 27 and 28 wheels must provide extra support to the back (non-grinding) face of the wheel, due to the additional stresses imposed by face grinding, particularly when the grinding is done with hand-held equipment. Therefore, the ANSI Standard requires that special mounting means be employed for Types 27 and 28 wheels, including a flange over the raised portion, extending beyond the raised area and in contact with the wheel to counteract side pressure. On the opposite side of the wheel, a flange must also be provided, contained completely within the depressed area.
The art has responded to these requirements by providing mounting means in the form of hub assemblies comprising several pieces, secured to the wheel by friction or resin bonding. The hub assembly in most general use comprises two parts. An adaptor includes a flange covering the raised area, having a mounting projection for attachment to the machine spindle and a sleeve extending through the mounting aperture. A flange nut is carried within the depressed area, with a sleeve extending into the mounting aperture to engage the adaptor, typically by threads. Such an arrangement depends entirely upon the nut being properly tightened, of course, and offers the potential for a high degree of hazard. Alternatively, the two pieces can be bonded to the wheel, as with epoxy resin.
Typical of proposed improvements in this art is U.S. Pat. No. 3,879,178, to Bosma. There, a tubular member extends through the wheel's central aperture, having a flange at its end in engagement with the depressed area of the wheel. A backpad is fitted over the tube to cover the raised area, and the tube is axially compressed, crimping it to the wheel. Clearly, this type of assembly not only is relatively cumbersome, but also it provides limited ability to counteract side pressure and to hold the wheel securely without slippage.
Other proposed solutions rely upon some form of mechanical interlock between the hub assembly and the wheel, to provide rotational stability. For example, U.S. Pat. Nos. 4,015,371, to Grayston, 4,240,230, to Ferrantini, and 4,541,205, to Patrello, all disclose hub assemblies having a sleeve extending through the wheel mounting aperture and crimped thereto, with some portion of the assembly mating with a specially-formed portion of the wheel. Grayston teaches the formation of indentations spaced around the aperture, receiving raised portions of the hub; Ferrantini offers similar indentations, but places them in the wheel periphery, engaged by bosses on the inner side of the flange rim; and Patrello employs bushings, formed into the wheel's mounting aperture during wheel manufacture, having a keyway to receive a key in the hub assembly. All of these approaches suffer from the defect of complicating the manufacturing process by requiring additional steps. Also, introduction of point loading areas, particularly in Grayston and Ferrantini, produces stress concentration areas that can lead to early wheel failure.
Additionally, the hub assemblies found in the art form a significant proportion of the total product weight. Given the reliance upon crimping and pressure fit, the art employs metallic hub assemblies, with the crimped modules generally being a zinc or aluminum alloy. Thus, the hub assembly leads to increased shipping costs. The art has heretofore failed to offer a grinding wheel hub assembly that is simple to manufacture, yet combines high safety with light weight.